In recent years, since optical discs can record a large amount of information signals with high density, they are used in many fields, such as audios, videos, computers and so on. Especially, the amount of data such as moving image information dealt with by computers etc. has been increasing drastically over the last years. With this, the capacity of optical discs has also been increasing by with a smaller recording pit and track pitch.
In the information recording medium, in order to reproduce information signals recorded by a micron unit, it is necessary to accurately track information tracks by optical beams. Although various methods have been known as a detection method of a tracking error signal (TES), the push-pull method is known as the simplest method. However, in the push-pull method, an offset is generated in TES when an objective lens shifts radially during tracking, or when the disc tilts. Then, as a method to cancel the offset, a method using three beams (differential push-pull (DPP) method) is disclosed in Japanese Patent No. 1756739 (published on May 13, 1986).
Moreover, in order to improve the DPP method and to omit a rotation adjustment for three beams on a disc, Japanese Publication for Unexamined Patent Application No. 250250/2001 (Tokukai 2001-250250, published on Sep. 14, 2001) proposes a new tracking method (will be referred to as “phase shift DPP method” hereinafter). This method is explained with reference to FIG. 31 to FIG. 34.
For example, as shown in FIG. 31(a), a laser beam from a semiconductor laser 1 is converted into a parallel ray with a collimator lens 2, and is divided into a main beam 30 a sub-beam 31 (+1st order component), and a sub-beam 32 (−1st order component) with a grating 3. After passing through a beam splitter 4, the three beams are condensed on a track 61 of an optical disc 6 with an objective lens 5. The reflected light of the three beams are reflected off the beam splitter 4 through the objective lens 5, and a condensing lens 7 guides the light to optical detectors 8 (8A, 8B, and 8C).
As shown in FIG. 32, the far field patterns of the reflected light of the main beam 30 and the sub beams 31 and 32 are received by the two-part optical detectors 8A, 8B, and 8C, which respectively have dividing lines corresponding to a track direction. The two-part optical detectors 8A, 8B, and 8C produce push-pull signals PP30, PP31, and PP32, respectively, which are differential signals.
Here, as shown in FIG. 31(b), an X-Y coordinate system is set up. The center of the beam is an origin, the radial direction of the disc is an x direction, and the track direction orthogonal to the radial direction is a y direction. In the grating 3, the phase of a periodic structure of the track groove in the first quadrant is 180-degree out of phase from that of other quadrants. Accordingly, the sub beams 31 and 32 diffracted by the groove portion have a 180-degree phase difference only in the first quadrant. Here, as shown in FIG. 33, the amplitudes of the push-pull signals PP31 and PP32 using the sub beams 31 and 32 become substantially 0 compared with the push-pull signal PP30 of the main beam that does not have a phase difference. This is because no push-pull signal is detected regardless of the position on the track and the signals are substantially the same regardless of whether the sub beams 31 and 32 fall on the same track as the main beam 30, or on a different track.
In contrast, as shown in FIG. 34, as for an offset of TES by objective lens shifting or disc tilting, the PP30 and PP31 (PP32) generate offsets Δp and Δp′ on the same side (in-phase) according to their respective light intensities. Therefore, by solving the following equation
      PP34    =                  PP30        -                  k          ⁡                      (                          PP31              +              PP32                        )                              =              PP30        -                  k          ·          PP33                      ,
a differential push-pull signal PP34 which has canceled the above offsets can be detected.
In the equation, the coefficient k is for correcting a difference in the light intensity between the 0 order component 30 and the +1st order component 31, and between the 0 order component 30 and the −1 order component 32. When the intensity ratio of 0 order component: +1st order component: −1st order component=a:b:b, the coefficient k=a/(2b). PP33 is the sum of push-pull signals of the sub beams 31 and 32. The principle as to why push-pull signals of the sub beams do not generate (amplitude 0) is omitted here. In this way, the amplitudes of push-pull signals of the sub beams become 0 regardless of the depth of the groove. That is, since the amplitudes are 0 wherever on a track the three beams locate, position adjustment (rotation adjustment of the diffraction grating, etc.) of the three beams becomes unnecessary, and assembly adjustment of the pickup can be simplified drastically.
Moreover, in the case that a hologram laser unit is used, a region actually swept by the sub beams is shifted from a region swept by the main beam on the diffraction grating, particularly when a phase-shifting diffraction grating is placed in the vicinity of the light source of the semiconductor laser. As a result, the two sub beams cannot have a common optimum phase shift. Even if it is possible to add an optimum phase shift pattern for the pitch or depth of a given optical disk, there remains a problem that it cannot be applied to an optical disc that has a different pitch. The foregoing conventional examples propose phase shift patterns that accommodate such a problem.
(Document 1)
Japanese Patent No. 1756739 (pages 1 to 4, FIGS. 1 to 3)
(Document 2)
Japanese Publication for Unexamined Patent Application No. 250250/2001 (pages 6 to 10, FIGS. 1 to 3)
However, in the case that the above phase shift grating is used, a region to which a phase shift is added needs to be optimally designed according to the push-pull pattern, i.e., the pitch or depth of the optical disc, the magnification of the optical system, or the NA (numerical aperture) used on a light source side. Therefore, the characteristics deteriorate when the width or position of the phase shift region shifts from the designed value due to fabrication error, or when the optical parameter of the pickup in which the grating is installed is changed.
Furthermore, even with the pattern that is designed to accommodate discs of different pitches, the conventional arrangements can only accommodate only two kinds of pitches. As a result, the characteristics deteriorate greatly when there is a pitch shift, or when a disc of a different pitch is used.
Moreover, the characteristics also deteriorate when a relative position of the objective lens shifts during assembly. A change in the objective lens shift characteristic during tracking is also large and there is a limit. Therefore, in the above conventional techniques, there are problems of versatility or mass-productivity, owning to the fact that they require small assembling tolerance, and individual phase shift pattern designing suitable for different optical systems or optical discs.
The present invention was made to solve the above problems, and an object of the invention is to provide a low-cost TES detection method using a phase shift DPP method, and an optical pickup employing same, (i) that can be installed in optical systems of optical pickups having different specifications, (ii) that are applicable to optical discs of different pitches or groove depths, and (iii) that undergo only small characteristic drop and improve objective lens shift characteristics even when there is a large assembly or design tolerance.